1. Field
The present disclosure relates to a ceria-based composition including bismuth oxide, a ceria-based composite electrolyte powder including bismuth oxide, a method for sintering the ceria-based composition or the ceria-based composited electrolyte powder, and a sintered body of the ceria-based composition or the ceria-based composited electrolyte powder.
2. Description of the Related Art
Electrolyte for use in sensors or fuel cells, etc., is an ion conductor through which ions generated at one electrode move toward the other electrode. Therefore, it is required for the electrolyte to have high ion conductivity and have no electron conductivity. In addition, when used in fuel cells, electrolyte is required to be so dense that a so-called cross-over phenomenon, in which anode gas is mixed with cathode gas, may be prevented, and to be stable structurally and chemically at high temperature and under both oxidative atmosphere and reductive atmosphere.
As for a material satisfying the above-described requirements relatively well, there is yttria-stabilized zirconia (YSZ). Yttria stabilized zirconia has an excellent mechanical strength and shows a stability and a reproducibility as an electrolyte of solid oxide fuel cells, etc., and thus is most widely used now.
However, manufacturing sensors or solid oxide fuel cells, etc. using yttria-stabilized zirconia electrolyte in large scale is difficult and requires high cost due to a high sintering temperature of about 1400° C. or higher.
Meanwhile, recently, studies have been conducted on electrolyte materials having high oxygen ion conductivity to provide high-performance solid oxide fuel cells. For example, studies have been conducted on bismuth oxide (Bi2O3), perovskite structured compounds such as lanthanum gallate (LaGaO3) or barium cerate (BaCeO3), ceria (CeO2), etc.
Particularly, among them, ceria has significantly high ion conductivity and relatively excellent mechanical properties, and thus is given many attentions as a prominent alternative electrolyte.
However, sintering ceria-based electrolytes is difficult and thus requires a higher sintering temperature (at least about 1500° C.) as compared to the known yttria-stabilized zirconia electrolyte. Moreover, ceria may be provided with electron conductivity when Ce4+ is reduced into Ce3+ under reductive atmosphere at an anode side, thereby causing a short circuit between a cathode and an anode. This makes it difficult to commercialize the ceria-based electrolyte.
Low-temperature sintering methods applicable to such electrolyte may include chemical vapor deposition (CVD), electrochemical vapor deposition (EVD), plasma sputtering, electrophoretic deposition (EPD), or the like. However, these methods require expensive equipments or processes, and thus are not suitable for scaling-up and cost saving.
Q. Zhu et al. discloses very fine particles with a size of about 9 nm obtained by using a hydrothermal process to reduce the sintering temperature of yttria-stabilized zirconia (Solid State Ionics 176, 889-894, 2005). Since a decrease in particle size results in an increase in surface energy, sintering of particles may be carried out at a temperature much lower than the conventional sintering temperature of bulk particles. However, the present inventors note that the above-mentioned method requires high cost to reduce the size of particles into several nanometers, which results in an increases of total manufacturing cost.
Zhang et al. discloses that incorporation of about 1% copper oxide or cobalt oxide to samarium-doped ceria may reduce a sintering temperature from about 1400° C. or more to near about 1000° C. (Journal of Power Sources, 162, 480-485, 2006). However, the present inventors note that the above-mentioned method still does not allow the sintering temperature to be reduced to about 1000° C. or lower. In addition, it is noted that even if a sintering aid agent (e.g. Co3O4, CuO, MnO3, etc.) is used in Zhang et al. to reduce the sintering temperature of ion conductive materials such as ceria, the sintering aid agent itself may serve as impurities so that it rather causes a degradation of the ion conductivity of the ion conductive material.